High-power, short-pulse laser irradiance can be used to write and store data onto a glass substrate. The irradiance induces, at its focus, long-lived or permanent structural and optical changes within the substrate medium, caused by non-linear (e.g., two-photon) absorption by the medium. In some cases, a nanoscale 3D structure with grating-like optical properties is formed at the focus of the irradiance. The term ‘voxel’ is used herein to refer to an individual locus of this or another induced structural or optical change within the substrate medium, which is useful for storing data.
A voxel can store data in many different forms. In principle, any of the Muller-matrix coefficients of a medium can be manipulated and used to encode data. In some examples, a voxel written onto a substrate may be modeled as a waveplate of a certain retardance δd (measured in nanometers (nm)), and angular orientation ϕ (measured in degrees). Both the retardance and the orientation may be used to encode data. When a voxel is written by a polarized laser beam, the angle of the polarization determines the orientation ϕ of the waveplate grating, while the intensity of beam (i.e., beam power or accumulated energy) determines the strength of the grating, and accordingly, the retardance δd.